Place the onion directly over a gas flame until scorched on all sides (alternately, heat a heavy skillet over high heat, cut the onion in half and scorch the onion in the skillet on all sides.

Transfer the onion to a large saucepan and add the broth, water, phớ spices, ginger, cilantro stems, fish sauce and salt and bring to a simmer. Simmer until broth is flavorful, about 20-30 minutes. Strain through a fine sieve and return to the pot to remain heated until ready to serve.

Bring a second pot of water to a boil. Drop the noodles into the boiling water and drain immediately; divide the noodles between 4 bowls.

Add turkey to the noodles. Fill the bowls with hot broth, top with scallions, more cilantro stems, bean sprouts, basil, and jalapeno slices.